In the forming section of a paper making machine, an aqueous slurry of fibers and fillers is deposited onto the paper side of the forming fabric. The machine side of the forming fabric is in contact with the static parts of the paper making machine. The forming fabric allows water to drain through, while retaining a proportion of the fibers and fillers on its surface so that a very wet paper sheet is formed.
The introduction of plastics monofilaments allowed forming fabric design to diversify in a manner that was not possible earlier. This invention is concerned with one of the resulting fabric types, namely composite forming fabrics.
Composite forming fabrics comprise two essentially separate single layer woven structures that are interconnected into a unified fabric, as described in U.S. Pat. No. 4,815,499. The paper side layer is optimised for sheet forming, and the machine side layer is optimised for stability and wear resistance.
A composite forming fabric should have the following features in combination:
1) excellent wear potential, through careful selection of yarn sizes and materials, fabric mesh and weave design; PA1 2) the yarns interconnecting the two layers should be removed as much as possible from the machine side area of wear; PA1 3) the machine side layer weave design should not induce lateral tracking of the fabric when in use; PA1 4) twinning of either or both the warp and weft yarns should be minimized by the weave design; and PA1 5) the fabric seam should be strong and non-marking. PA1 a) a paper side layer of interwoven warp and weft yarns; PA1 b) a machine side layer of interwoven warp and weft yarns; and PA1 c) interwoven binder yarns binding the paper side layer and the machine side layer together into a unified structure; PA1 i) the warp and weft yarns are woven according to a repeating multiple shed pattern which is an N by 2N weave in which N is the number of sheds and is an integer from 3 to 12 inclusive; PA1 ii) the multiple shed pattern provides at least two distinct warp yarn floats having different float counts within one repeat of the weave pattern; PA1 iii) the multiple shed pattern provides at least one distinct weft yarn float within one repeat of the weave pattern; and PA1 iv) all of the weft yarn floats are located substantially in a single plane and are exposed on the machine side of the machine side layer. PA1 a) a paper side layer of interwoven warp and weft yarns; PA1 b) a machine side layer of interwoven warp and weft yarns; and PA1 c) interwoven binder yarns binding the paper side layer and the machine side layer together into a unified structure; PA1 i) the warp and weft yarns are woven according to a repeating six-shed 6.times.12 pattern which provides: PA1 ii) all of the weft yarn floats are located substantially in a single plane, and PA1 iii) all of the weft yarn floats are exposed on the machine side of the machine side fabric layer.
The term "wear potential" refers to the amount of yarn material that may, on average, be abraded away from the exposed machine side yarns before fabric replacement is necessary. In all forming fabrics the machine side of the fabric is exposed to abrasive wear, which will erode the machine side yarns to a point where those yarns become so thin that they are unable to provide the required tensile strength. The forming fabric must then be replaced. Any increase in the wear potential of a forming fabric is highly desirable.
Although increasing the size of the yarns in the machine side layer will generally increase fabric wear potential, this can result in a relatively thick forming fabric which tends to retain excess amounts of water.
It is also possible to increase the wear potential by increasing the length of the weft floats exposed on the machine side of a composite forming fabric. The float length can be increased by increasing the number of sheds in the machine side layer weave, and hence 3-, 4-, and higher shed designs are progressively "better" than 2-shed designs. Further, with higher shed numbers it is easier to obtain a large crimp differential. The term "crimp differential" refers to the essentially vertical distance between the most prominent warp yarn knuckle and the most prominent weft yarn knuckle. The value of the crimp differential is indicative of the order in which the machine side yarns begin to wear, and the amount of wear that is available. As the crimp differential increases, both mechanical stability and wear potential increase. The practical limit on float length in forming fabrics that are in commercial use today has been 4. Most composite forming fabric machine side layers are woven in 3, 4 or 5 sheds, with 5 shed satin designs being preferred. Designs that use 6 sheds, or more, have not been applied to the machine side layer of composite forming fabrics.
Composite forming fabrics having a machine side layer woven as a 3 or 4 shed twill exhibit "lateral tracking" and tend to drift laterally in use on the paper making machine in the direction of the twill, thereby increasing the difficulty of guiding the fabric in a straight run.
Another problem with composite forming fabrics having a machine side layer woven as a 4 shed cross or broken twill is that either the warps, or the wefts, have pronounced tendencies to pair, or twin. This reduces alignment and registration of the paper side and machine side yarns. The resulting different sized drainage passages adversely affects paper quality.
The manner in which the two layers of a composite forming fabric are interconnected also has an impact on wear potential. Failure of the interconnecting yarns results in delamination of the two layers. Two interconnection methods are used: additional binder yarns, or "intrinsic yarns". The chosen yarns can be either warps or wefts.
Additional binder yarns are yarns interwoven between the machine side and paper side layers during manufacture to bind them together. These binder yarns are usually of relatively smaller diameter than the machine side layer yarns, and will fail rapidly if exposed to abrasive wear. To minimize wear, the binder yarns are recessed as much as possible within the machine side layer structure.
"Intrinsic yarn" comprises an existing yarn, that already forms a portion of the paper side layer weave. The paths of selected yarns are modified so as to pass through both fabric layers. Intrinsic weft yarns are particularly suitable when the stock contains a relatively high amount of particulate filler material.
Although warp binder yarns are attractive because they are more economical, incorporating them into a composite forming fabric presents two difficulties to the manufacturer.
The paper side or machine side warp binders may cause discontinuities in the paper side surface, especially when the paper side layer is a plain weave and intrinsic warps are used. Second, if the machine side and paper side layer weave designs are quite different, the path lengths of adjacent or proximate warp yarns may not be identical. This method is more problematic and therefore is not generally preferred by manufacturers.
The seam is a weak point of any forming fabric, particularly when wear levels are high. Seams for flat woven forming fabrics are most often woven back seams. A high strength, non-marking seam is particularly difficult in composite forming fabrics.